1. Field of the Invention
The present invention relates to a signal compression, and more particularly, to a signal compressing apparatus which controls output signal level depending on input signal level to increase transmission efficiency.
2. Discussion of the Related Art
A background art signal compressing apparatus will be described with reference to the accompanying drawing.
FIG. 1 is a block diagram illustrating a background art signal compressing apparatus.
As shown in FIG. 1, the background art signal compressing apparatus includes a resistor 11, an operational (OP) amplifier 12, and a gain controller 13.
The resistor 11 is connected between an input terminal 14 and an input terminal of the OP amplifier 12. The output of the OP amplifier 12 is connected to an output terminal 15.
The gain controller 13 is a feedback path of the OP amplifier 12 and is connected between the input of the OP amplifier 12 and the output thereof.
The gain controller 13 includes a resistor 13a, a capacitor 13b, a rectifier 13c, a band pass filter 13d, and an operational transconductance amplifier (OTA) 13e.
The band pass filter 13d is connected between the output of the OP amplifier 12 and the input of the rectifier 13c.
The output of the band pass filter 13d is connected to the input of the rectifier 13c, and the resistor 13a and the capacitor 13b are connected to the output of the rectifier 13c. Also, the output of the rectifier 13c is connected to the input of the OTA 13e.
The output of the OP amplifier 12 and the output of the rectifier 13c are input to the OTA 13e. The OTA 13e is connected to the input (current summing node) of the OP amplifier 12.
The operation of the aforementioned background art signal compressing apparatus will be described below.
As shown in FIG. 1, an audio input signal transmits current corresponding to an input signal to the current summing node, which is the input of the OP amplifier 12, through the resistor 11. The gain controller 13 forms a feedback path of the OP amplifier 12. The gain of the gain controller 13 is determined by ratio of effective resistor and the input resistor 11. Therefore, the effective resistor value of the gain controller 13 is controlled to be a gain predetermined by the size of the output.
The size of the output signal can be varied depending on the size of the input signal by controlling the effective resistor value of the gain controller in advance depending on the size of the output.
The band pass filter 13d of the gain controller 13 serves to limit the frequency range of the output signal.
At this time, the minimum cut-off frequency and the maximum cut-off frequency of the band pass filter 13d are set as the value for increasing signal-to-noise ratio of the signal compressing apparatus. Also, the minimum cut-off frequency and the maximum cut-off frequency are set in order to improve rectifying characteristic of signal by RC time constant of the capacitor 13b and the resistor 13a.
The rectifier 13c rectifies the output signal of the OP amplifier 12 passing through the band pass filter 13d using RC time constant by the resistor 13a and the capacitor 13b.
At this time, DC signal passing through the rectifier 13c is input to the OTA 13e.
Therefore, the OTA 13e varies its gain by the input DC signal. Thus, the output of the OP amplifier 12 is converted to current depending on the varied gain of the OTA 13e while passing through the OTA 13e, so that it is transmitted to the current summing node, i.e., the input of the OP amplifier 12.
However, in the background art signal compressing apparatus, although there is provided a filter at a feedback path of the OP amplifier to improve noise characteristic and a signal suitable for time constant of the rectifier to improve rectifying characteristic, the background art signal compressing apparatus has several problems.
First, if exceeding input signal is applied, the output of the OP amplifier becomes exceeding, thereby causing distortion of the output signal at low voltage operation.
Second, if the size of the output becomes larger in order to improve signal transmission efficiency, dynamic range of transmission increases, thereby reducing transmission efficiency.
Finally, if there is provided an automatic level controller (ALC) in order to above problems, temperature characteristic of rectifying voltage is sensitive to affect the output signal, so that it is impossible to obtain stable output.